In many applications it is necessary to provide an observer with a substantially undistorted view through a multi-layered laminate structure. In one common application, two substantially transparent rigid substrates, such as sheets of glass or a transparent polymeric material, must be bonded together with an adhesive layer between them. Or, one of the substrates may be relatively rigid and the other flexible. For example, an imaged polymeric film may be adhered to a glass panel as a decorative decal. A polymeric film may be adhered to a screen of a display device to take advantage of the polarizing, or reflective properties of the film to increase the brightness, color, or contrast of the display.
To apply a polymeric film to a screen of a display device, a window, or a vehicular windshield, heat and/or photo curable adhesives are not practical. In these applications pressure sensitive adhesives are traditionally used to bond the substrates and form the laminate. Pressure sensitive adhesives do not require a separate curing step like heat or photo curable adhesives, and may be more easily removed and/or repositioned on the substrate.
However, when the substrate and the pressure sensitive adhesive layer are adhered, it is difficult to ensure a firm and reliable bond in the laminate structure. In addition, air is typically trapped at the interfaces between the adhesive and the substrate, and the resulting bubbles cause haze and compromise the optical properties of the laminate. It is inconvenient and messy to wet a substrate with water or a plasticizer to control adhesion and allow trapped air to dissolve into the adhesive layer at the interface with the substrate.
The pressure sensitive adhesive backed articles described in U.S. Pat. No. 5,897,930, U.S. Pat. No. 6,197,397, WO 00/69985, and U.S. Ser. Nos. 09/311,101, 09/638,346 and 09/639,244 include an adhesive layer with a plurality of interconnecting microchannels. A typical article 10 of this type is shown in FIG. 1, and includes a film 12 having opposed surfaces 14 and 16. The surface 14 of the film 12 is imaged to form a graphic 13. A layer of a pressure sensitive adhesive 18 is bonded to the surface 16 of the film 12. The pressure sensitive adhesive 18 includes a surface 20 that can be bonded to a substrate. A release liner 26 is releasably attached to the pressure sensitive adhesive 18. The release liner 26 includes protrusions 28 and land areas 29 that form corresponding channels 24 and land areas 22 in the pressure sensitive adhesive layer 18. The release liner 26, shown in a partially removed state, is fully detachable when pulled in the direction of arrow A and is used to protect the pressure sensitive adhesive prior to application of the article 10 on a substrate.
The geometry and spacing of the protrusions 28 and land areas 29 on the release liner 26, and the corresponding channels 24 and land areas 22 in the pressure sensitive adhesive layer 18, may be selected, along with the surface characteristics of the substrate and the rheology of the adhesive itself, to provide specific temporary and/or permanent properties for the article 10. In general, the geometry and spacing of the structures in the release liner and adhesive layer should be sized according to the following general design considerations. First, the protrusions should preferably be sufficiently large to allow egress of fluids to the periphery of the adhesive layer for exhaustion into the surrounding atmosphere, but not so large as to allow ingress of unwanted fluids beneath the adhesive layer. Second, the structures should also not be so large as to detract from the appearance of an exposed surface of a film adhered to the adhesive layer, particularly if the film is to be imaged. Third, the structures should not be so large as to detract from the adhesive bonding performance of the adhesive layer.